1. Field of the Invention
The present invention relates to eye examining and testing instruments such as eye refractometers, retinoscoper and eye fundus cameras.
2. Description of the Prior Art
To effect an accurate measurement or inspection of the human eye and also to take a good picture thereof, it is of importance that the testing instrument be correctly aligned and spaced relative to the eye. If the alignment is incorrectly made, then a substantial error will be included in the obtained value of the measurement with a refractometer. Also, in case of a fundus camera, the light used for taking a picture may be adversely reflected by the iris of the eye. On the other hand, incorrect spacing will cause various problems. Usually, spacing is made by structurally predetermining the distance of a face holder on which the face of the patient is held motionless, from the objective lens of the instrument. For fundus camera, there is required a particularly accurate adjustment of the distance between the eye and the objective lens which distance is generally called the working distance. If the adjustment of working distance is incorrect, then a portion of light for illuminating the fundus may be reflected by the cornea into the picture taking light so that a flare may be produced in the image. An accurate adjustment of spacing, if attained, will greatly serve to increase the accuracy of measurement with a measuring instrument such as refractometer.
In the specification of U.S. Pat. No. 3,871,772 there is given a description of the positioning of an eye examining instrument. According to the adjustment method of alignment or spacing described therein, the anterior eye is uniformly illuminated with infrared light through an objective lens while observing the illuminated anterior eye with an aiming device supported on the body of the instrument. By aligning the center of the aiming plate with the center of the pupil of the eye to be measured, an adjustment of alignment and spacing is achieved. However, this known method has several drawbacks. In the view field of the aiming device there appears the whole anterior eye. Furthermore, the resolving power of the image is reduced because of an image intensifier incorporated into the aiming device. Therefore, it becomes very difficult for the inspector to judge whether the image of the anterior eye appearing in the view field is sufficiently sharp and clear. Thus, both the adjustments of alignment and spacing must be carried out depending only upon the positional deviation of the center of the pupil from the center of the aiming plate. This makes it impossible or very difficult to carry out the adjustment of aligning and that of spacing independently from each other.